Material processing machines, and in particular extruders, comprise several parallel screws having threads whose pitch can vary along the axis and penetrate one into the other so as to draw the material along the threads by rotation of the screws. For this purpose, the screws are placed inside a casing with several lobes, each lobe having a cylindrical inner wall enveloping one screw over the portion of the periphery of the latter outside the penetration zone.
To control the conditions of advance of the material in the screws, it is generally necessary for the clearance between the periphery of the screw and the inner wall of the casing to be as small as possible. For this reason, the inner walls of the casing must be bored with precision and, in machines comprising several screws, the constitution of the casing in several lobes renders this operation delicate and laborious.
In addition, the mass of the casing and its inner wall must have different and often contradictory characteristics since they are subject to stresses which are not at all the same in depth and at the surface. In fact, in material processing machines, the casings are subject to thermal stresses due to operation between 150.degree. and 300.degree. C. and mechanical stresses created by the pressure of the extruded product thrust by the screws, which pressure can exceed 200 bars. On the other hand, the inner wall undergoes wear caused by the extruded product and by the friction of the screws and/or corrosion.
All these stresses consequently impose on the casing the need for particular and conflicting properties, since the outside must have moderate strength matched with good resilience and the inner wall must possess high resistance to fatigue cracking, high hardness, a low coefficient of friction and a high resistance to abrasion.